The following relates to ultraviolet (UV) light emitting diodes (LED's), UV lasers, and related technologies.
Many materials have been developed as hosts for elements with optically active energetic transitions to produce lasers and electroluminescent devices with emission in the visible and infrared (IR) part of the electromagnetic spectrum. These include electrically driven devices utilizing rare earth (RE) phosphors in wide or medium gap semiconductors, such as thin film electroluminescent devices (TFED) with RE phosphors which have found applications as visible and IR emitters.
Optoelectronic devices which utilize atomic transitions in the rare earths for electroluminescence (EL) can offer a number of advantages over EL produced from band-to-band recombination. First, the transition energy is dictated by the energy level scheme of the 4f orbital, which is relatively unperturbed by the crystalline environment due to the fact that the 5d and 5s,5p orbitals extend to further radial distances, and fill before the f shell of lower principal quantum number. This shielding by the earlier filled 5d and 5s orbitals causes the energies of the 4f transitions to be relatively insensitive to crystalline imperfections, unlike transitions based on band-to-band transitions in semiconductors. Band-to-band optical transitions are sensitive to deep levels, exciton-phonon interaction, and crystalline disorder which can lead to broadening of emission or parasitic emission at an unintended energy. Additionally, due to the decoupling of the 4f orbital with the lattice, emission from rare earth centers is spectrally pure with common full width at half maximum (FWHM) of less than 30 meV.
The spectrally narrow and energetically stable nature of the Gd3+ fluorescence emission make it a potential candidate for spectroscopic and lithographic applications in the UV. This has led to exploration of dilutely Gd-doped AlxGa1-xN in the form of fluorescence and cathodoluminescence experiments. Although the 4f levels in the RE3+ are typically thought not to interact with the surrounding lattice, cathodoluminescence data for Gd:AlN thin films show phonon replica satellite peaks of the Gd3+ 6P7/2→8S7/2 (318 nm) transitions. Vetter et al., Appl. Phys. Lett. 83, 2145 (2003). These data suggest that the 4f electrons in Gd3+ in AlN are not completely decoupled from the host lattice. Other Gd:AlGaN compounds spectroscopy are reported in: Kita et al., Appl. Phys. Lett. 93, 21190 (2008); Zavada et al., Appl. Phys. Lett. 89, 152107 (2006); Gruber et al., Phys. Rev. B 69, 195202 (2004).
Less work has focused on development of active optoelectronic devices that utilize Gd3+ 4f transitions. One difficulty is achieving electrical contact to unintentionally doped (uid) AlN. In one approach (see Kita et al., Appl. Phys. Lett. 93, 21190 (2008); Kitayama et al., J. Appl. Phys. 110, 093108 (2011)), a field emission device consists of a reactive ion sputtered AlxGd1-xN film with metal contacts, forming a metal-insulator-semiconductor (MIS) structure whereby a high voltage on the order of 270 volts to greater than 1000 volts driven across the device produces fluorescence of the Gd3+ ions, likely by the process of impact excitation.